Pyrolytic carbon, which is referred to herein as pyrocarbon, is generally deposited by thermally decomposing a gaseous hydrocarbon (or other carbonaceous substance) in vapor form as a coating upon relatively small substrates that can be levitated in a fluidized bed along with an ancillary charge of small particles. In the time that has passed since the development of fluidized bed technology for applying pyrolytic carbon coatings, as exemplified for example in U.S. Pat. No. 3,977,896 to Bokros et al. entitled “Process for Depositing Pyrocarbon Coatings”, it has been discovered that there are many variables with respect to the fluidized bed environment that may determine the structure of the pyrocarbon that is deposited. Because pyrocarbon (having a microstructure that is free of growth features) is generally deposited when the relative amount of deposition surface area to volume in a fluidized bed is fairly high, such small objects to be coated with pyrocarbon may be tumbled in a bed of minute particles. In many cases, the substrates being coated, which are larger than the minute particles, will exhibit substantially random motion within the fluidized bed. As a result, the surface area of the substrate will generally be substantially equally exposed to the upward flow of the mixture of hydrocarbon and inert gas flowing through the coating enclosure. Consequently all or most surfaces will receive a substantially uniform thickness of pyrocarbon. However, when such relative uniformity of tumbling motion does not occur, possibly because of the particular geometry of the substrates being coated, uniformity of coating thickness may not be achieved.
Specifically, the configurations of some objects or substrates have a tendency to assume a relatively stable orientation in an upwardly flowing fluidized bed despite collisions with other substrates and the particles in the bed. This tendency is referred to as a tendency to “plum-bob” (i.e., to float in one particular orientation within the bed, and not randomly tumble). When such plum-bobbing occurs, an inadequate coating thickness may be applied to surface regions that are essentially shielded or hidden from direct contact with the upwardly moving hydrocarbon stream and “free carbon”. This may be because the shielded surface regions experience pyrolysis and deposition of pyrocarbon at a slower rate. The result of such a situation can be an unacceptably thin deposition of pyrocarbon coating in these regions and therefore overall non-uniformity of pyrocarbon thickness across the entire surface of the substrate. For instance, a thin hemispherical shell with a stem protruding axially from within the concave surface (e.g., an orthopedic implant) may float within the coating bed with the convex face facing downward receiving a desired amount of coating while the base of the stem (where it affixes to the concave face of the shell) is predominately shielded from the coating and receives an unacceptably thin deposition of carbon.